Journal of Water Resources Engineering
Year:2013 | Volume:6 | Issue:17|Papers Count:8

Most rivers have flood plains that extend laterally away from the main channel at a gentle gradient or in a series of terraces. Multistage channels are deliberately formed in certain cases in order to increase conveyance capacity in large floods, and to have recreational expanses available at other times of the year. Two-stage channels, thus consist typically of a main river channel in which there is some discharge all of the time and flood plains, which are dry for most of the time, yet perform a vital function in times of flood. Since flood alleviation schemes are the focus of much of engineering work, the prediction of the conveyance capacity, velocity distribution and boundary shear stress distribution in such channels is clearly important. The boundary shear stress distribution is a prerequisite for studies on bank protection and sediment transport. Prediction of these parameters in two stage or compound channels is complicated by the lateral exchange of momentum that takes place in the shear layer that forms between the generally faster moving water in the main river channel and the slower moving water on the flood plain. Superposition of high lateral shear on bed-generated turbulence and longitudinal secondary flow structures is a convoluted problem in fluid mechanics. In the context of the river channels with flood plains, the problem is usually further complicated even for moderately straight channels by the complex geometry of the cross-section and the heterogeneous nature of the boundary roughness.

Different hydraulic and hydrological methods are used in flood routing for construction of the downstream hydro graph of a river. Hydraulic flood routing methods, initially based on the Saint-Venant equations, are developed using the continuity equation and employing the terms of the momentum equation, consist of three models: kinematic wave, diffusion wave, and dynamic wave. The objective of this study was to evaluate the accuracy of the kinematic wave and dynamic wave methods. This project was implemented in a 3.5 km reach of the Doab samsami River, which is a tributary of the Karoon River. Fourteen cross sections were surveyed in the reach and their Manning's roughness coefficients were estimated based on the expert judgment. A computer program was developed for flood routing with the kinematic wave method. For the dynamic flood routing, the unsteady flow analysis of the HEC-RAS software was used. The developed hydro graphs using the two methods were compared with the actual hydro graph at the downstream station using the RMSE criterion. Results indicated that the kinematic wave and the HEC-RAS models were very close, indeed. Therefore, based on the simplicity of the method, and also the limited data requirement of the kinematic wave method, its use is strongly recommended for similar studies, and also for flood routing in Rivers with larger gradients.

The mean annual precipitation (MAP) is the most important criterion used in drought classification. Different indices have been developed based on the MAP for drought classification, of which four most commonly used are: the percentage indices of normal (PN), the standardized precipitation index (SPI), deciles (DPI) and the rainfall anomaly index (RAI). Each of these indices is classified into classes for drought description. Each class is represented as a state of severe drought. This study compares these indices benefiting from the rainfall data collected during 30 years (1980-2010) at seven stations in the Province of Sistan andfsaluchestari, After calculating the indices during the desired time scale, the results were classified and evaluated based on sequence similarity factors of dry, normal and wet years, and by using the cluster analysis and developing statistical correlation between them. The analysis indicated that the PN index and RAI were statistically similar and lead to identical results in the dry and hot climate of Sistan and Baluchestan. It was further revealed that, the paired PN-RAI, PN-SPI and SPI-RAI were highly correlated at most of the stations. Moreover, the PN index and the RAI were placed in one class in the cluster analysis. Calculations indicated that the results of PN and RAI criteria were close to each other in terms of describing drought. Therefore, these indices are recommended for determining the drought severity in the province of Sistan and Baluchestan. Despite the widespread use of the SPI, this index is not suitable for determining the severity of drought in this province.

A new probabilistic approach was adapted to the negative impact of climate on stream flow. To generate climate change scenarios in the future under the A2 emission scenario, the HadCM3 model was employed. By introducing climatic variable time series in future periods to the IHACREShydrologic model, long-term stream flow simulation scenarios were produced. By fitting statistically different distributions on the runoff produced by using a goodness-of- fit test, the- most appropriate statistical distribution for each month was chosen and relevant statistical parameters were extracted and compared with statistical parameters of the runoff in the base period. Results show that the long-term annual runoff average in the three future periods compared to the base period. Despite the reduction in the total runoff volume in the future periods will decrease compared to the baseline period, the decrease is related to the medium and high flows. In low flows, the total runoff volume for future periods compared to the baseline period will increase 47, 41, and 14%, respectively. To further assess the impact of annual average runoff on flows, it is necessary to examine correlation of time series using the stream flow transition probability. To compare the stream flow transmission probability in each of the future periods with base period stream flow in each month, stream flow was discretized and performance criteria were used. This approach was adapted for the Aidoghmoush River, East Azerbaijan. Results indicated a low coefficient of correlation and a high error indicator.

Soil infiltration rate is the most important factor in designing irrigation systems. Therefore, selection of a suitable infiltration equation is a prerequisite for achieving the best performance of any irrigation system. As a plant's performance integrates numerous factors, particularly water demand in arid environments, providing adequate and timely water in irrigated fields passes a challenge. In this regard, the coefficients of three popular infiltration equations, namely: Kostiakov-Lewis, Philip and SCS, were investigated along the volume balance equation. The cumulative infiltration depths in furrows planted with sweat com were also compared using ' different equations. Three periods: first, mid and end of maize growth season were considered in this study. Results indicated that the application of Philip equation (one point method) was not applicable because it over-predicted the infiltration depths. Moreover, the mean absolute error (AAE) of the Philip equation (0.00683) and SCS equation (0.00373) were larger than that of the Kostiakov-Lewis equation (0.00253). Finally, the results revealed that the Kostiakov-Lewis equation were the best predictor of the three infiltration equations used in this study.

Stepped spillways are an accessory of reservoirs and water conveyance system, which dissipate flow energy and reduce the size and costs of stilling basins (SBs). As the reverse inclination of each step helps form a small SB, therefore, energy dissipation becomes more efficient. In this study, using a Plexiglas spillway (height: 72 em), which was installed in an experimental flume (width: 50 em), various arrangements of numbers and inclination steps of spillway were investigated installing 12 set-ups. The first experiment was conducted on the weir itself without any steps; another set-up was with one step without inclination and one step with reverse inclination. This procedure was continued asymmetrically with the array of 8 steps. As expected to efficiently dissipate the energy, more steps are required as the flow increases. It was observed that one inclined plus two horizontal steps provide the best energy dissipater array. Furthermore, it was revealed that there was a limit to increase the number of steps and their inclination in energy dissipation, i.e., efficiency may not be enhanced by increasing the number of inclined steps.

Different measures may be taken into account in order to reduce the destructive effects of flow over spillways, one of which is the application of stepped spillways in an attempt to dissipate the flow energy. This method also influences the economic justification of the project and reduces the costs. The present study simulated the free surface and the two-phase flow patterns over the stepped spillway employing the Fluent software, and adopting the mixture model as well as the standard K - E turbulence model. The numerical model verification was conducted utilizing the data of air concentration distribution in water, and the velocity at different discharge rates collected in experiments performed by Chamani utilizing a physical model at Hydraulic Laboratory conducted at the University of Alberta. The numerical simulation results and experimental data were in good agreement. Concentration profiles led to a stair like distribution of the concentration, and the discharge effects on the shapes of velocity profiles were comparable. The results showed that as the inlet discharge increased, velocity profiles widened and spread more, and flow height and maximum flow velocity rose, which in turn reduced the flow dissipation.

Water-use-efficiency, which is a matter of utmost importance in irrigated-agriculture in the water-short Iran, requires installation of accurate flow measurement devices on the outlets of delivery channels. Accuracy, low cost, and ease of installation and measurement are the criteria for a good measuring device. As a long-throated flume with a broad-crested weir cross-section had been studied in detail by Zand-Parsa, and the Win Flume software was available for the design of-the flume and processing of the collected data, 10 such flumes were installed in 3, third degree channels in the Dorudzan Irrigation District in 2007, however, this study was implemented in the 2012 irrigation season, using 3-5 replication for each flume set up. Both our collected data and the Win Flume program validated the equation developed by Zand -Parsa. It was observed that the percentage error in measuring low flows ranged 12-21, which was much higher than that of the maximum flow? Based on these results, the large scale use of this device and the application of the Zand-Parsa equation are recommended.